This invention relates generally to turbine components and more particularly to a combustor liner that surrounds the combustor in land based gas turbines having can annular combustion systems.
Traditional gas turbine combustors use diffusion (i.e., non-premixed) combustion in which fuel and air enter the combustion chamber separately. The process of mixing and burning produces flame temperatures exceeding 3900 degrees F. Since conventional combustors and/or transition pieces having liners are generally capable of withstanding for about ten thousand hours (10,000 hrs.), a maximum temperature on the order of only about 1500 degrees F., steps to protect the combustor and/or transition piece must be taken. This has typically been done by film-cooling which involves introducing relatively cool compressor air into a plenum formed by the combustor liner surrounding the outside of the combustor. In this prior arrangement, the air from the plenum passes through louvers in the combustor liner and then passes as a film over the inner surface of the liner, thereby maintaining combustor liner integrity.
Because diatomic nitrogen rapidly disassociates at temperatures exceeding about 3000xc2x0 F. (about 1650xc2x0 C.), the high temperatures of diffusion combustion result in relatively large NOx emissions. One approach to reducing NOx emissions has been to premix the maximum possible amount of compressor air with fuel. The resulting lean premixed combustion produces cooler flame temperatures and thus lower NOx emissions. Although lean premixed combustion is cooler than diffusion combustion, the flame temperature is still too hot for prior conventional combustor components to withstand.
Furthermore, because the advanced combustors premix the maximum possible amount of air with the fuel for NOx reduction, little or no cooling air is available, making film-cooling of the combustor liner and transition piece premature at best. Nevertheless, combustor liners require active cooling to maintain material temperatures below limits. In dry low NOx (DLN) emission systems, this cooling can only be supplied as cold side convection. Such cooling must be performed within the requirements of thermal gradients and pressure loss. Thus, means such as thermal barrier coatings in conjunction with xe2x80x9cbacksidexe2x80x9d cooling have been considered to protect the combustor liner and transition piece from destruction by such high heat. Backside cooling involved passing the compressor air over the outer surface of the combustor liner and transition piece prior to premixing the air with the fuel.
With respect to the combustor liner, one current practice is to impingement cool the liner, or to provide linear turbulators on the exterior surface of the liner. Another more recent practice is to provide an array of concavities on the exterior or outside surface of the liner (see U.S. Pat. No. 6,098,397). The various known techniques enhance heat transfer but with varying effects on thermal gradients and pressure losses. Turbulation strips work by providing a blunt body in the flow which disrupts the flow creating shear layers and high turbulence to enhance heat transfer on the surface. Dimple concavities function by providing organized vortices that enhance flow mixing and scrub the surface to improve heat transfer.
There remains a need for enhanced levels of active cooling with minimal pressure losses and for a capability to arrange enhancements as required locally.
This invention provides convection cooling for a combustor liner by means of cold side (i.e., outside) surface features that result in reduced pressure loss.
In the exemplary embodiment of this invention, discrete ring turbulators are provided on the cold side of the combustor liner, each ring defined by a circular raised tubular shaped rib enclosing an interior area or hollow interior region. The ring turbulators are preferably provided as a uniform staggered array over substantially the entire cold side surface of the liner. In one arrangement, the ribs have a square cross-section, but the cross-sectional shape may vary to include, for example, rectangular and tapered inside and/or outside edge surfaces. The edge surfaces may also vary about the periphery of the ring, dependent on the direction of cooling air flow. In addition, the inside and outside corners of the ribs may be sharp or smooth. Ring type turbulators maintain many of the positive effects of known linear turbulators, but the rounded shape and the xe2x80x9cconcavexe2x80x9d areas enclosed by the ring will produce lower pressure loss. The round shape of the turbulators still disrupts the flow, but does so in a manner which is more distributed, especially if the rings are patterned in a staggered fashion. At the same time, the xe2x80x9cdimplexe2x80x9d or xe2x80x9cbowlxe2x80x9d shaped interiors form the vortices for fluid mixing. Thus, heat transfer enhancement is by both turbulated effect and dimpled effect.
The height and width of the ribs may also vary, and the xe2x80x9cfloorxe2x80x9d of the enclosed area may be raised above the outer non-ring surface area of the liner.
Accordingly, in its broader aspects, the invention relates to a combustor liner for a gas turbine comprising a substantially cylindrical body having a plurality of raised ribs arranged on an outside surface of the combustor liner, each rib defining an enclosed area on said outside surface.
In another aspect, the invention relates to a combustor liner for a gas turbine comprising a substantially cylindrical body having a plurality of raised circular turbulator rings arranged on an outside surface of the combustor liner; and wherein the rings have height and width dimensions of between about 0.020 and 0.120 inches and inside diameters of between 2 and 5 times the height dimension.
In still another aspect, the invention relates to a method of cooling a combustor liner in a gas turbine combustor comprising establishing a flow path for compressor discharge air along an outer surface of combustor liner; and forming a plurality of discrete ring turbulators on the outer surface of the combustor to enhance heat transfer, each ring turbulator comprising a raised peripheral rib of substantially round or oval shape.